Comet Dirt

Credits: ESA/Rosetta/Philae/ROLIS/DLR

It looks, someone responded when I posted the above to Twitter, like my back yard. It's actually the surface of Comet 67P/Churyumov–Gerasimenko, taken from a distance of nine metres by the Philae lander, at the very end of its descent after being released by the Rosetta comet chaser. The lander failed to anchor itself and in the comet's feeble gravity bounced off the surface several times, ending up wedged in the shadow of a low cliff. Without enough sunlight falling on its solar panels the little lander went into sleep mode after its battery power ran down, but recently woke again because the intensity of sunlight has increased as the comet makes its closest approach to the sun. A short while ago it transmitted a fat batch of data, just published, including that close-up of its first, very temporary landing site. Which does, yes, look like a patch of garden dirt. Or maybe the hardcore of a building-site car park. Or the surface of Mars, or of the Moon. Which either suggests (if you hate the idea of space exploration) that travel to other planets is a waste of time, or (if, like me, you geek out on planetary science) says something interesting about the universality of dirt. That there are similar geological processes on comets and planets that grind bedrock fine, and with the aid of gravity and wind (or the eruptive jets of comets) distribute the material in a more or less even blanket. That a comet isn't a simple ball of ice, but possesses dirt and boulders, cliffs with mass-wasted talus slopes, and even what look like rippled dunes.

But even outwith the fact that it's part of the rind of a comet, the dirt in the image isn't ordinary dirt, of course. It's mostly water ice. Pebbles and shards and grains of water ice frozen hard as rock, leavened with carbon dioxide and carbon monoxide ices, and tainted with a variety of toxic chemicals - hydrogen sulfide, hydrogen cyanide, sulphur dioxide, carbon disulphide, formaldehyde, methyl isocyanate, acetone, propionaldehyde, acetamide . . . 'If you could smell the comet, you would probably wish that you hadn't,' as one of the Rosetta team wrote in the project's blog. But in that poisonous cocktail are compounds that probably played key roles in Earth's ancient prebiotic chemistry. You couldn't grow flowers in comet dirt (although if you were one of the Quiet War's Outers, you might seed a comet like this with vacuum organisms that would mine useful organics), yet it contains the stuff of life: stuff that may have seeded the Earth with necessary precursors. That patch of comet dirt is a reminder of where we came from.